Ultrabroadband coherent supercontinuum frequency comb

Abstract

We present detailed studies of the coherence properties of an ultra-broadband super-continuum, enabled by a new approach involving continuous wave laser sources to independently probe both the amplitude and phase noise quadratures across the entire spectrum. The continuum coherently spans more than 1.5 octaves, supporting Hz-level comparison of ultrastable lasers at 698 nm and 1.54 µm. We present the first numerical simulation of the accumulated comb coherence in the limit of many pulses, in contrast to the single-pulse level, with systematic experimental verification. The experiment and numerical simulations reveal the presence of quantum-seeded broadband amplitude noise without phase coherence degradation, including the discovery of a dependence of the supercontinuum coherence on the fiber fractional Raman gain.Highly spatially and temporally coherent super-continua (SC) based optical frequency combs have applications ranging from broadband spectroscopy with high sensitivity and accuracy [1], to the spectral dissemination of optical frequency references, where coherently linking the visible spectrum to the telecom band is crucial for long-haul optical carrier transfer [2]. Additionally, and most importantly, highly phase coherent frequency combs directly impact the field of optical frequency standards, where the SC generated with phase-stabilized frequency combs allows comparison of optical frequency standards hundreds of THz apart [3]. Currently, the thermal noise-limited frequency stability of continuous wave (CW) cavity-stabilized lasers sets a limit to the frequency stability of these optical standards, especially to those based on trapped ensembles of neutral atoms [4]. A direct spectral link between the 1.5 µm and visible spectral regions allows novel ultra-stable laser systems operating in the telecom band [5] to be used as the local oscillators for current optical standards. A direct combbased approach offers more complete spectral coverage and can circumvent frequency doubling of the CW laser or frequency comb, which adds non common-path noise in addition to complexity.In this Letter, we present a new method for verifying supercontinuum optical coherence, which allows us to conclusively demonstrate a spectrum that directly and coherently spans from the visible to the telecom band. This technique, applied to an ultra-broadband spectrum 1.5 octaves wide, utilizes optical heterodyne beat experiments and numerical simulations. This powerful combination has allowed us to identify regions of significant quantum-seeded amplitude noise that are decoupled from * Present address: Institute for Lasers, Life and Biophotonics, Vrije Universiteit Amsterdam, The Netherlands; aruehl@few.vu.nl † Present address: COMSET/ECE, Clemson University, Clemson, SC 29625 the phase noise of the continuum, proving that the comb phase coherence is preserved across the entirety of the 1.5-octave spectral range. In concert with numerical simulations, we have reached an understanding of the important role that fiber Raman gain plays ...

High-power frequency comb source tunable from 27 to 42 μm based on difference frequency generation pumped by an Yb-doped fiber laser

“…This technique is also commonly used as a coherence test of SC [27] and DFG sources [15]. Since no MIR CW laser was available, we frequency-doubled the MIR idler centered at around 3100 nm using the 1-mm thick PPLN, resulting in ~3 mW of SHG radiation.…”

Femtosecond coherent seeding of a broadband Tm:fiber amplifier by an Er:fiber system

“…These results are very encouraging, since a significant loss of coherence has been reported recently for a similar setup, especially in the longwave solitonic part of an SC generated in a microstructured fiber pumped with an Yb:system [3]. The explanation for the superior coherence performance of our approach is as follows: the HNFs are cut to be shorter than the characteristic fission length, L fiss , of the soliton.…”

“…Other approaches based on well-established Er:fiber technology may be even more desirable. However, Raman self frequency shifting [10] or supercontinuum (SC) generation in microstructured fibers [11] turned out to be ineffective, since they provide seed pulses flawed by a significant degree of incoherence [3,12].In this Letter, we present a passively phase-locked and ultrabroadband source that is suitable for coherent seeding of both Yb: and Tm:fiber systems. This setup exploits Er:fiber technology combined with frequency conversion in highly nonlinear bulk fibers (HNF).…”

“…Examples include materials processing [1], nonlinear microscopy [2], and precision metrology [3]. A new frontier for femtosecond fiber systems is represented by broadband seeding of various high-power amplification schemes.…”

“…Other approaches based on well-established Er:fiber technology may be even more desirable. However, Raman self frequency shifting [10] or supercontinuum (SC) generation in microstructured fibers [11] turned out to be ineffective, since they provide seed pulses flawed by a significant degree of incoherence [3,12].…”